Del Potro, the losing finalist here last year, is ranked number five in the world, compared with Roger-Vasselin's standing of 189.

"I didn't play my best tennis," del Potro lamented after the match.

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"I came here very close to the competition. Maybe I needed a few more days to be in shape, but the other guy played very good tennis, he took his opportunities, he (played) a very good match."

Roger-Vasselin fought evenly against the Argentine in the first set until the ninth game when del Potro double-faulted to give a 15-40 chance to the Frenchman.

Del Potro charged to the net on a short ball, only to see Roger-Vasselin fire a sizzling forehand cross-court passing shot to move one break up and then served out the set.

Roger-Vasselin set up another 15-40 chance in the fifth game of the second set, which he won when del Potro's forehand landed in the net.

The Frenchman saved all six break points he faced and notched up his first victory over one of the world's top five-seeded players.

Del Potro blamed the loss on jet lag and a stomach problem. "Of course, I didn't play good tennis and that's the reason (I lost)."

Roger-Vasselin, 25, said confidence helped him gain the upper hand.

"I think the first reason was that I believed I could win. I said on the court 'why not today? Maybe he's not very confident in the first round,' so I took my chances. That's it," he said.

"I was a little bit tense at the beginning, my first-game serve was difficult, but after that I thought maybe I can beat him, because when there were rallies, I won the points," he said.

"I served very good, I had a few break points on my serve, and every time I played a good serve, sometimes aces, sometimes serve and volleys. This is the key of the match. I did very well when I had break points on my serve."